Otto Heckmann was educated at the University of Bonn, worked at the University of Göttingen from 1927 to 1941, and directed the Hamburg Observatory from 1941 to 1962. He determined positions and proper motions of the faint members of the Praesepe cluster and worked on the structure and dynamics of several clusters. He made major contributions toward producing the second Astronomische Gesellschaft Katalog (AGK) to show proper motions of 180,000 stars, and he organized the international effort to produce the third. He and other observatory directors then established observatories in the southern hemisphere to produce a similar catalog for the southern skies. At Hamburg he helped design and oversaw the installation of a 1.2-m Schmidt telescope. Equally adept at observation and theory, Heckmann made significant contributions to cosmology: he found open and flat solutions to the Einstein field equations for a homogeneous, isotropic universe, and he introduced the possibility of a rotating universe. His book on cosmology rigorously developed the consequences of general relativity. After World War II Heckmann joined with other leaders in European astronomy to promote the idea of a joint observatory in the southern hemisphere, and from 1962 to 1969 he served as the first director general of the European Southern Observatory.